Technical Abstract:
Cattle serve as a clinically silent reservoir for the shiga-toxigenic human pathogen Escherichia coli O157:H7 (STEC O157), and control of this organism is important both for beef production and for public health. STEC O157, which colonizes the gastrointestinal tract, can be isolated from cattle feces, hides, and saliva, as well as environmental samples such as water and soil. Pest flies found within the cattle production environment, including stable flies (Stomoxys calcitrans), horn flies (Haematobia irritans), face flies (Musca autumnalis), and house flies (Musca domestica) have the potential to be mechanical vectors of STEC O157. In fact, STEC O157 has been isolated from farm-associated fly pools, and laboratory experiments have shown that this pathogen can survive and grow in M. domestica for at least three days. In order to examine whether flies can serve as a vehicle for transmission of specific STEC O157 strains in the cattle production environment, we used Pulsed Field Gel Electrophoresis (PFGE) molecular fingerprinting to characterize the relationship between STEC O157 strains of pest fly and animal origin in two distinct environments: state and county fairs, and individual dairy or cow-calf operations.
Animal STEC O157 isolates were obtained from feces, hide or mouth swabs, or mouth rope specimens. Fly isolates were collected from whole live fly pools and sorted by species. A total of 53 STEC O157 isolates from fly pools and livestock at two fairs were typed using PFGE, as were 155 STEC O157 isolates from fly pools and fecal samples obtained from several livestock operations. All specimens were enriched for 6 hours at 37°C in appropriate media, then isolated using Dynal IMS beads plated on Chrom agar. Isolates were confirmed as STEC O157 by ELISA and by PCR. The Pulse Net Standardized Laboratory Protocol for Molecular Subtyping of Foodborne Bacterial Pathogens by PFGE was used for all isolates, and fingerprint data was analyzed using Bionumerics software.
Results from the state and county fairs show a diversity of PFGE profiles in both livestock and fly pools, indicating multiple STEC O157 genotypes exist in this environment. However, there were indistinguishable PFGE profiles from livestock isolates and fly pool isolates from the same fair. Results for the livestock operations generally showed a narrower clonal profile than the state fair isolates, with most fly and cattle isolates from any single farm on a given day demonstrating nearly identical PFGE patterns.
The isolation of identical PFGE types from livestock and fly pools at fairs and on farms confirms the significance of cross-species contamination or common source exposure taking place in the livestock environment, and demonstrates that flies can carry the same pathogen genotypes as livestock. These results provide evidence for a role of pest flies in the transmission of STEC O157, however the directionality of STEC O157 transmission between flies and cattle cannot be determined.